Australian Stringhalt

By Dr John Kohnke BVSc RDA

Australian Stringhalt is used to describe the involuntary and exaggerated upward flexion or ‘puppet-like’ action in one or both hind legs as a horse walks, often starting as a slight inco-ordination in gait and developing into a ‘goose’ stepping movement in severe cases, making it difficult for the horse to walk, graze or exercise. Although all breeds of horses can be affected, Thoroughbreds, perhaps because of their numbers in retirement, appear to be more susceptible, with ponies being the least susceptible because they eat less bulk of feed.

Stringhalt is often a seasonal condition that develops in one or more horses in a group grazing sparse, unimproved pastures after the break of the season in Summer or Autumn, usually following a few days of rain and warm, humid weather.

Underlying Causes

Although the Stringhalt condition, as occurs in other countries can be caused by neurological damage to the hind limb nerves by microbial infections and trauma, Australian Stringhalt and its seasonal occurrence is more specific to Australia and New Zealand, especially following drought conditions. Often it affects specific localities or geographical areas due to favourable seasonal conditions. Research in the late 1980’s and early 1990’s in Australia and New Zealand was carried out to find the possible cause for the damaging effect on the long nerves of the hind limbs and neck. These studies concluded that horses grazing pasture dominated with the weed known as Cats Ear or Flat Weed (Hypochoeris radicata) were associated with seasonal outbreaks of Stringhalt.

The type of nerve damage suggests a mould toxin (‘mycotoxin’) or a fungal ‘poison’ that directly affects the long myelinated nerves in the hind limbs and also the long left Recurrent Laryngeal Nerve in the neck. Severe cases develop the characteristic ‘goose-stepping’ gait and often a ‘roaring’ like sound when exercised due to collapse of the left laryngeal structure.

Typical Symptoms

Observations indicate that it takes 7-21 days of grazing Flat Weed dominated pastures to cause early symptoms of Stringhalt, with slight inco-ordination in one or both of the hind limbs when the horse is initially walked off, which improves as the horse ‘warms up’. In grazing horses affected by Flat Weed, often they can be seen walking around with early symptoms of gait inco-ordination, which often becomes exaggerated when these horses become excited, chased away from feed or are caught to be ridden or worked. Horses that are thin and hungry are more likely to be affected early in a group of horses. In more severe cases, horses exhibit mild signs when quiet, but develop total inco-ordination and ‘goose-stepping’ gait when agitated or excited, especially by the inability to walk properly. Often horses are unable to be backed without severe inco-ordination and risk of falling over, and may be difficult to unload after travelling.

Management

If both back legs are affected, horses develop a ‘bunny-hopping’ like gait and cannot be exercised. When stringhalt occurs after the break of the season where pastures contain Flat Weed, the horses should be removed from the pasture to a pasture free of any Flat Weed or into a holding area and hand fed hay. Dampened lucerne hay is considered the best roughage as it contains higher energy, protein and minerals than grassy hay, along with a vitamin supplement such as Cell-Vital^® or Cell-Provide^®.

It is of no use to feed hay to horses left to graze the contaminated pasture as they will continue to eat Flat Weed.

Excitement often exaggerates the ‘goose-stepping’, puppet-like hind limb inco-ordination, so it is helpful to keep the horse quiet and avoid working it until the symptoms improve and the horse can walk comfortably.

If the signs are recognised early, often improvement occurs over a 2-3 week period, with recovery in 2-3 months. More severe symptoms will often improve over 6-12 months but some cases take 18 months and do not fully recover. In the chronic condition, loss of muscle on the outside of the gaskin area and weakness in the fetlock joint on one or both hind limbs can complicate recovery.

Where a horse develops a ‘roaring’ on inspiration during exercise, it should be scoped to determine the severity of the left vocal cord collapse.

Medical Treatment

Although drug therapy with the anti-convulsant drug phenytoin, can hasten recovery in some horses, long term therapy may be required. Another central acting nerve drug, baclofen, is also effective in some cases, with a noticeable response within 7-10 days of starting therapy, with continued improvement after ceasing therapy in some horses, even with symptoms over 12 months in duration. Unfortunately, other horses show little improvement despite long term therapy.

Dietary Supplementation

Supplementation with the mineral magnesium has been associated with improvement in early cases of Stringhalt.

However, it appears that not all sources of magnesium, such as in Dolomite, which is poorly absorbed in horses, and Epsom Salts (magnesium sulfate) are effective.

Numerous reports suggest that supplementation with Mag-E, containing large amounts of organic (chelated) magnesium proteinate, combined with natural and synthetic Vitamin E and Vitamin B1, can assist nerve and muscular function in affected horses. Supplementation with 3 scoopsful of Mag-E daily for a 500kg horse for an initial 5-7 days, then 2 scoopsful daily for 3-4 weeks, may help in improving signs, combined with rest and removing horses from the Flat Weed dominated pasture. A longer duration of supplementation may be of benefit in some horses to speed up recovery.

Prevention

The control of Flat Weed with selective herbicides will reduce the risk of poisoning. However, establishment of more competitive, vigorous improved pastures for grazing horses is the best long term strategy to reduce the spread of Flat Weed.

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